Tailoring intermetallic precipitation to achieve ultrahigh strength and low thermal expansion in Super Invar alloys

초록

In Fe-Ni-Co-based Super Invar alloys, achieving both ultrahigh strength and low thermal expansion is challenging, as precipitation strengthening enhances mechanical strength but simultaneously depletes solutes from the matrix, thereby compromising the composition stability required for the Invar effect. Two model alloys, Fe-40Ni-4Co-1.5Al-2Ti (wt.%) and Fe-40Ni-4Co-2Ti (wt.%), were designed to promote gamma ' (L1(2)) and eta (D0(24)) precipitation, respectively, while maintaining coefficients of thermal expansion (CTE) below 3 ppm K-1. AT-A developed dual-mode gamma ' precipitation, consisting of uniformly distributed intragranular spherical precipitates (similar to 15 nm) and nanolamellar-like precipitates along the grain boundaries, both of which reduce strain incompatibility and prevent stress localization. T-A also contains intragranular spherical gamma ' precipitates but develops needle-like eta precipitates along the grain boundaries, where the semi-coherent interfaces of gamma (FCC) and eta precipitates cause severe strain mismatch and stress concentration. Consequently, AT-A achieved a yield strength of 933 MPa, ultimate tensile strength of 1158 MPa, and 18% elongation, surpassing T-A (840 MPa, 1022 MPa, 13%) due to finer lamellar spacing of precipitates along grain boundaries. CTE reduction in both alloys was quantitatively predicted by the rule of mixtures, highlighting that precise Al/Ti ratio control enables a robust pathway to simultaneously achieve high strength and dimensional stability in precipitation-strengthened Super Invar alloys.

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